A SIP network is a communication network using the session initiation protocol (SIP). In a session initiation protocol (SIP) network a SIP-client, which is connected to an access network e.g. a fixed access network, may be connected behind a network address translation (NAT) server. A NAT-server may be a digital subscriber line (DSL) modem, which is part of an access network. The DSL modem is connected to a core network via a digital subscriber line access multiplexer (DSLAM).
Between the access network and the core network a session border controller may be located in order to control the connections, which have access to the core network. In particular the core network is an internet protocol (IP) network.
A session border controller may be known from the ETSI-TISPAN (European Telecommunications Standards Institute—Telecoms & Internet converged Services & Protocols for Advanced Networks) specification ETSI-TS 183 017. The command format for diameter based commands used by the Gq′ interface is described in ETSI specification ETSI-TS 183 017.
The structure of the SIP messages (e.g. Re-INVITE, 200_OK, ACK) is described in IETF's (Internet Engineering Task Force) RFC 3261, and the structure of a SDP body is described in RFC 2327. The session description protocol (SDP) is used for the description of multimedia sessions over IP-based networks. SIP may rely on SDP. SDP attributes are known from IETF's RFC 2327.
The message format for a H.248 message or a H248 message used by the Ia interface is described in ETSI specification ETSI-ES 283 018.
An end device, a user equipment or a user terminal using the SIP protocol is called a SIP-client. A plurality of SIP-clients may be connected by SIP/SDP-sessions or SIP/SDP-connections. Media streams may be exchanged via a SIP/SDP-session. After establishing a SIP/SDP-session between two or a plurality of SIP-clients the SIP/SDP-session may be routed via a NAT-server. The NAT-server monitors media streams which are exchanged between the connected SIP-clients.
The NAT-server determines when a media stream is idle for a certain period of time, which means that for that period of time no user data have been exchanged or transmitted via the media stream or the media stream connection. When such an idle state is detected, the NAT-server closes the relevant media stream by closing pinholes within the NAT-server, which are associated with corresponding media stream.
In other words, when the NAT-server determines an inactive media stream, the NAT-server closes the connection using the functionality of closing the pinholes. As a consequence, the whole connection is shut down. Once a NAT-server has closed a media stream between the SIP-clients, no further exchange of media data or media streams between the SIP-clients will be possible. When the media streams between the SIP-clients have been shut down by the NAT-server, no possibility exists to bring the connection up again besides establishing a new connection.
Such a shutdown of a media stream connection may also happen when a SIP-client has suspended a connection for a certain time. A suspension of a connection may be required when a SIP-client wants to put a connection on hold or put a call on hold. The function of putting a connection on hold quite often is called a call-on-hold function. There may be a need for preventing a shutdown by the NAT-server during a call-on-hold function.